feat(commands): reply to /help with the commands and labels

[mike-diff]

Closes #35

Summary

Why: a user who comments /help gets the command list, the workflow work label and needs-human in a reply, with a 👍 ack and no change to labels, state or hold, so they no longer need the README or the CLI --help.

Changes

 commands.go      | 21 ++++++++++++++++++++-
 commands_test.go | 34 ++++++++++++++++++++++++++++++++++
 2 files changed, 54 insertions(+), 1 deletion(-)

Status

Step	Result	At
Checks	pass	864d33d
Judge	approve	864d33d
Repairs	0

[ghafk]

Approved at 864d33d

#	Result	Evidence
1	pass	go build ./... && go vet ./... exited 0.
2	pass	gofmt -l . printed nothing.
3	pass	go test ./... reported ok for all three packages.
4	pass	go test ./... -run 'TestParseCommand' passed. commands_test.go has case {"/help", "help", ""}, and parseCommand
5	pass	go test . -run 'Help' -v ran TestHelpRepliesWithCommandsAndLabels and passed. The test posts a trusted OWNER /help
6	pass	manual. I read commands.go, comment.go, workflow.go, commands_test.go and README.md. steerIssue handles `ver